A multi-substrate assembly package may contain several die packages and multiple die systems or modules in a package. System in Package (SiP) is a number of integrated circuits enclosed in a single module. Multiple dies may be stacked vertically or horizontally connecting to a SiP interposer. An interposer comprises a multi-layer substrate that interconnects the active areas on the top-side of the interposer with the active areas connected to the back-side of the interposer. The multiple dies may be internally connected by fine wires that are bonded to the interposer or package. Alternatively, with flip chip technology, solder bumps are used to join the die to the SiP interposer. Despite the benefits of multichip modules including SiPs, problems such as warpage of a SiP module interposer may create challenges during integration of the module into a multi-substrate assembly.
The module interposers may be thin in comparison to the surface area on which components are mounted. Some interposers are made with dimensionally stable material, such as ceramics, however, many interposers are comprised of materials such as multiple layers of Cu, polyimide or Cu, fiberglass resin (FR4), and the like, that are semi-rigid and may flex during process temperatures. The flexibility of the interposers may allow the module packages to warp in the axis perpendicular to the major surface areas or in the Z dimension.
Ball-grid array (BGA) packages, for example, ideally have a rectangular mounting surface that is generally planar. The mounting surface may be covered with small spherical leads that carry electric signals to and from the active areas that are a part of the multiple integrated circuit module. However, under certain conditions, an SiP BGA interposer may warp causing a warped mounting surface on the BGA module. A warped BGA module may create bridged or open solder joints between the SiP BGA substrate and a printed circuit board (PCB) surface of the multi-substrate assembly package.
Further, warpage of a module may create significant measurable differences from solder joint to solder joint across the surface of an assembly package. First, as discussed above, the substrate material may warp resulting in a significant variation in the relative distances between the mounting surfaces of the substrate and the PCB. Generally, BGA packages warp such that the edges tend to pull away from the mounting surface of the PCB. However, it should be appreciated the BGA substrate warp can occur in a host of various ways with the distance between the mounting surfaces of a BGA substrate and a PCB varying in a number of different ways across the mounting surfaces. Other packages, such as, but not limited to, column grid array (CGA), flip-chip, chip scale packages (CSPs), and quad flat packs may also warp. Secondly, a difference in non-shorted or non-bridged solder joints may be problematic because of solder columns with irregular diameters causing reliability concerns.
These and other factors may work together such that a multi-substrate assembly may contain a plurality of multi-pin devices with a significant variation in one or more solder joint measurements. For example, in the case of solder columns in which a substantially uniform volume of solder paste is applied to the mounting surface of the assembly PCB, an increase in the distance between the mounting surfaces of the BGA module and the PCB may cause one or more solder columns to open. A decrease in the distance between the mounting surfaces of the BGA module and the PCB may cause a bridge between two adjacent solder columns. An open or bridge may likely cause a failure of the workpiece. Prior art methods of working around these problems include limiting module size, therefore limiting module warpage, limiting module materials, such that all components of the module have closer coefficients of thermal expansion, and scrapping warped modules. A disadvantage of the prior art is that warped packages bonded to a PCB may have open or bridged solder joints.
What is needed then, is a new method and structure for adapting solder volume to a warped substrate that overcomes the above described shortcomings in the prior art.